Described below is a method that relates to Internet communications technology, in particular, an IP address allocation method and its usage.
As well as providing traditional data communications, broadband Internet is also ideal for VoIP (voice telephony). VoIP can provide clients with many practical and effective functionalities, including lower business costs, unifying voice and data communications, and consolidating Internet management platforms, etc. VoIP is a telephony application which operates through TCP/IP (Transfer Control Protocol/Internet Protocol) protocols over the IP net. VoIP technology is currently not only hugely successful on the fixed Internet, but is making steady progress on the mobile Internet, especially in emerging 3G services, and it is forecast that VoIP will become the main technology for telephony. VoIP mobile terminals can access the Internet via WLAN (Wireless Local Area Network). WLAN uses wireless communications technology to establish an Internet connection within a specified area, and is a product of the combination of computer network and wireless communications technology. It can transfer media over multiple wireless communications channels and provide traditional fixed-line area network capabilities, providing users with effective broadband Internet access anytime, anywhere. Within a WLAN network, wireless terminals fitted with wireless connection cards use wireless access points (AP) as hubs to connect the wireless local area network to the fixed-line network via wireless bridges, wireless access gateways, wireless access controllers, wireless access servers, etc., which can create a variety of sophisticated wireless local area network access routes, and provide wireless mobile office access.
When a VoIP mobile terminal (MT) hands over among different IPv4 subnets, the MT must apply for a new IP address (IP address re-allocation) to effect handover from one IPv4 subnet access point to another. Before the MT receives this new IP address, any conversation over the MT must be interrupted. This requires the IP address re-allocation process to be fast enough (less than 20 ms) to prevent extended interruption to voice conversations, and reduce call quantities.
Current technology normally uses DHCP (Dynamic Host Configuration Protocol) to implement high-efficiency dynamic IP address allocation. DHCP is based upon the typical client/server model, where the client initiates requests, and the server replies with the appropriate reply. In this case the client is a normal computer, and the server is a DHCP server—when the computer instructing or requesting an address from a DHCP server by issuing an address request, the DHCP server automatically provides the client with an IP address and other Internet parameters, and dispatches a reply. As can be seen from FIG. 1, the DHCP service procedure is as follows:    1. The identification phase, where the DHCP client is seeking a DHCP server. The DHCP client in broadcast mode (because the DHCP server's IP address is unknown to the client) emits “DHCP discover” signals, seeking a DHCP server by sending specific broadcast information to 255.255.255.255. All Internet hosts on which the TCP/IP protocol is installed will receive this broadcast signal, but only DHCP servers will be able to reply.    2. The allocation phase, where the DHCP server allocates the IP addresses. Any DHCP server in the Internet will reply to a “DHCP discover” signal, allocate one of its unallocated IP addresses to the DHCP client, and send an allocation signal including the allocated IP address and other “DHCP offer” data.    3. The selection phase, where the DHCP client selects the IP address provided by a DHCP server. If several DHCP servers dispatch “DHCP offer” signals to the DHCP client, the DHCP client will only accept the first “DHCP offer” signal that it receives, and then replies in broadcast mode with “DHCP request” signal, which contains the IP address which it requested from the DHCP server that it selected. This reply in broadcast mode then notifies all DHCP servers that it has selected the IP address provided by that specific DHCP server.    4. The confirmation phase, where the DHCP server confirms the IP address that it provided. When the DHCP server receives the “DHCP request” signal reply from the DHCP client, it sends a “DHCP ack” acknowledgement which contains the IP address that it allocated and other defined to the DHCP client, to confirm to the DHCP client that it may use the IP address that it provided. After that, the DHCP client then binds its TCP/IP protocol to the network card, and all other DHCP servers except for the server which the DHCP client selected then retrieve the IP address which they allocated.    5. Re-registration. Whenever the DHCP client subsequently re-registers on the network, it no longer needs to transmit a “DHCP discover” signal, but can directly transmit a “DHCP request” signal containing its previously allocated IP address. When the DHCP server receives this signal, it will attempt to allow the DHCP client to continue to use the original IP address, and reply with a “DHCP ack” acknowledgment signal. If this IP address cannot be re-allocated for use by the original DHCP client (for example where the IP address has already been allocated to another DHCP client), the DHCP server replies to the DHCP client with a “DHCP nack” denial signal. Upon receiving this “DHCP nack” signal, the DHCP client must then re-transmit a “DHCP discover” signal to request a new IP address.
However, using the DHCP protocol for dynamic address allocation has one drawback: DHCP cannot identify the IP addresses in use by non-DHCP clients on the network, and when there is more than one DHCP server operating on one network, a DHCP server cannot verify the IP addresses already allocated by other servers. For these reasons, the DHCP protocol requests the client to use ARP (Address Resolution Protocol) to validate the allocated IP addresses. ARP uses transmitted signals to receive the MAC addresses for IP addresses on the network. The client transmits broadcast packages to all hosts on the same section of the network, so that when any host using the same IP address receives this package, it will send a reply signal to the client. In multiple applications, the ARP response time definition exceeds 1 second, while DHCP requests the client to wait at least 10 seconds before initiating another application. During this time, the MT thus has no allocated IP address, and any conversation on the MT will be interrupted, which will affect communication quality.